Existing electrophotographic printer technology makes use of a photoconductive drum. Depending on the type of photoconductor used, the drum is either charged or discharged to attract toner, with the charging or discharging accomplished by exposing the drum with light. The drum then transfers the toner to the paper or other media to be printed.
To expose the drum, various imaging devices can be used. Lasers scan the drum pixel-by-pixel. Spatial light modulators (SLMs) can expose an entire line (or a number of lines) at one time. Both types of imaging devices are digital in the sense that, at some point prior to being printed, the image to be printed is represented by pixel data in bit-map form. In other words, the image is logically divided into pixels, each pixel having a pixel value that is one or more bits in resolution.
The data received by the printer is typically page description data, which the printer rasterizes into pixel values having a certain bit-resolution. The higher the bit resolution, the more greyscale levels that are represented. The actual printing capability of a particular printer may permit the same or a different number of greyscale values. Various techniques have been developed for printing greyscale. For acceptable printing, at least 256 greyscale levels is considered acceptable.
One approach to printing greyscale levels is to vary the number or intensity of exposures of each pixel, thereby controlling the amount of toner at the corresponding location on the drum. This in turn determines the perceived darkness of the "dot" printed by that drum location. For example, in the case of a laser printer, the light intensity can be modulated. In the case of an SLM, a series of micro-images can be used, with each micro-image indicating whether each element of the SLM is to be on or off during a line period. This method results in "multi-level" digital values per pixel, but the number of available levels is limited to less than 256 levels.
Another approach to greyscaling is known as "screening". This method can be used to increase the greyscale levels beyond the per-pixel capability of a particular printer. In general, the image is screened into cells of m.times.n pixels. Each pixel in the cell is given a greyscale value that will result in a certain perceived greyscale value for the cell. This is because the eye integrates the pixels in the cell so as to perceive a single "spot".
Screening can be used with printers whose per-pixel capability is only black or white or with multi-level printers. As an example of the former, if an image were screened into cells having a size of 4.times.4 pixels, and eight pixels in the cell were on and eight were off, the printed "dots" corresponding to that cell would be perceived as having a greyscale value between black and white.
When screening is combined with multi-level capability, the technique is known as "multi-level screening". For example, suppose a printer can provide 16 greyscale levels per-pixel. If a cell has 4.times.4 pixels, one pixel is printed at level 15, another at level 10, and the other 14 at level 0, the cell would have a perceived greyscale of 25. In this manner, the cell can represent 241 greyscale levels, where 241=(16-1).times.4.times.4+1.
Some printer systems have improved image quality when the screening results in a concentration of exposures on the page. For example, in one-dimensional "line-screening" the entire image is screened into diagonal lines and the exposures are concentrated along these lines. The result is a less "noisy" image, especially at low intensity levels. Screening can also be multi-dimensional, such as by clustering exposures around perpendicular lines or around the midpoint of the cell. Assigning pixel values in this manner is also referred to as "modulating" the data.
The greyscale method and modulation determine the bit resolution and the particular values of the pixel data. However, it is often necessary to quantize the pixel data to a smaller bit resolution. For example, a printer might be capable of printing 8-bit pixel data, but its exposure unit might have a limited input bandwidth. In this case, the data could be quantized for input to the exposure unit and dequantized within the exposure unit for printing. Or, to reduce memory requirements, it might be desired to store only 4-bit data. In this case, the image data could be quantized to 4-bit data, stored, and then dequantized prior to printing.
When quantization is performed in a printer that uses modulated data, existing quantization methods are performed as part of the screening process. When pixels are assigned their modulated greyscale values, a look-up table assigns a modulated value having the quantized bit resolution. The same look-up table is used for all pixels.